This invention relates to the field of dental implants, more particularly, to the components used in dental implant systems and, most particularly, to the screws which are used to assemble such systems.
Dental implants are the subject of many patents and extensive literature. Artificial roots are implanted in the jawbones of patients and used to support replacement teeth. The tooth may be fastened directly to the root or it may be fastened to an intermediate part, called an abutment. In most systems, small screws are used to connect the parts. The screws which are used to connect the abutment to the implanted root typically have minor diameters of about 0.055-0.059 inch (1.4-1.5 mm). Retaining screws, which hold the tooth to the abutment part, may have minor diameters of about 0.0419-0.0453 inch (1.06-1.15 mm). Such screws are made of various metals and alloys, particularly, palladium, titanium and gold alloys, which are biocompatible and have become accepted for dental use.
It will be apparent that when such implanted artificial teeth are used to chew food (mastication), they are subject to significant forces. These forces place loads on the screws holding the tooth and any abutment to the implanted root. While those screws are intended to prevent the components of the implant system from separating, the mastication loads may cause the contacting surfaces of the components to open slightly on one side of the implant system by bending one or more of the screws. This creates what will be referred to herein as a "microgap," which typically occurs at the interface between the opposed surfaces of the abutment and the implanted root. Oral fluids may gain access to the interior of the implant system through the microgap, risking infection. Movement of the implant components may also cause the screws to loosen or fail as they are repeatedly stretched and bent. Avoiding the forces on the implant system is not within the control of the implant designer or the dentist who installs the implant. What they can do, however, is pretension the screws to attempt to prevent the forces encountered during use from causing separation of the individual components of the implant system.
As a screw is fully threaded into a prethreaded bore, the screw is tensioned between the engaging threaded surfaces of the screw and the bore, and the abutting surfaces of the screw head and the stationary seating surface around the bore. After the screw head seats on a stationary surface, the tension on the screw increases as the screw is threaded farther into the bore. This tension on the screw produces a force that is commonly referred to as the "preload" of the screw.
Classical screw theory relates the degree (angle) of turn of a screw to preload or clamping force by the following simplified equation: EQU F=(P.theta./360)K
where:
F=preload or clamped force of the two parts held together by the screw (e.g., the abutment to the implant), PA1 P=pitch of the abutment screw (e.g., 0.4 mm for a typical abutment screw), PA1 .theta.=degree (angle) of turn measured after snugging of screw head against opposed surface (i.e., abutment/implant surfaces are seated together), and PA1 K=spring constant of the screw and joint.
If the degree of turn (.theta.) is increased, the resulting clamping force (F) is also increased. An increase in the clamping force results in a tighter abutment/implant joint. The tighter joint imparts greater resistance to screw loosening and increases the load required to pry the abutment/implant joint apart. Side loads produced during mastication result in forces that tend to pry the abutment/implant joint apart. Joint prying and fatigue strength are directly related and, thus, the greater the force required to pry the joint, the greater the force required to cause cyclic fatigue failure of the screw.
In general, the fatigue strength of the screw increases as the preload increases because the screw remains more stable when subjected to various loads. The farther a screw is threaded into its bore after seating of the screw head, the greater the preload on the screw, i.e., the greater the force exerted by the inherent resilience (elastic recovery) of the screw itself on the opposing surfaces responsible for the tension on the screw. Advancing movement of the screw into its bore is resisted in part by the friction between the rotating surfaces of the screw and the opposed stationary surfaces, which must be overcome by the applied torque to advance the screw. By reducing the friction between the rotating surfaces of the screw and the opposed stationary surfaces, the preload on the screw can be increased for any applied torque because that torque will cause the screw to be advanced farther into its bore.